Materials 5: Polymers, ceramics and composites Flashcards
What properties can be obtained from an impact test?
1) The amount of energy absorbed by a material during fracture. The energy required to fail a material is related to the area under the true stress-true strain characteristics. Metals with high strength and high ductility have good toughness while ceramics and some composites have poor toughness due to their poor ductility despite having high strength. 2) Impact testing can be used to determine the transition temperature of materials; i.e. the temperature at which the material changes from brittle (low temperature) to ductile (high temperature) failure. Hence if a material is likely to be subjected to impact loading while in-service a material should be selected which has a transition temperature well below its in-service temperature minimum.
How is a typical fracture toughness test performed?
All materials contain flaws to one degree or another - these occur during synthesis, processing, abuse and in-service use. These flaws can lead to stress concentrations or weaknesses within the material depending on the number, size and geometry of these ‘defects’. Fracture toughness can be used to determine the influence of these flaws on the material to withstand applied loads A typical test to determine fracture toughness of a material is to apply a stress to a sample, which has a known flaw in terms of size and geometry, inserted in it. The applied stress is intensified by the presence of the flaw, which acts as a stress raiser
What factors allow materials to resist crack growth?
Larger flaws reduce the load bearing stress in a material. Therefore during manufacture impurities are filtered from liquid metals and hot pressing of ceramic components reduces the flaw size and hence enhances fracture toughness. Increased ductility increases fracture toughness. Increasing the strength of a metal decreases ductility and hence reduces fracture toughness i.e. there is a trade-off in properties. Brittle materials such as ceramics and some polymers have much lower fracture toughness than many metals. Thickness of materials influences the fracture toughness with thicker more rigid materials having lower fracture toughness than thin materials. The rate of load application, e.g. impact testing, reduces the fracture toughness of materials. Increase of temperature increases fracture toughness . A small grain size improves fracture toughness. More point defects (especially large) and dislocations reduce fracture toughness. Hence a fine-grained ceramic material helps to improve resistance to crack propagation.
Describe how a fatigue test is performed?
Components are often subjected to repeated application of stress below the yield strength of the material. After a significantly large number of cycles failure can result, despite the loading being well below the yield strength. E.g. Wings of an aircraft where the structural components are subjected to cyclical loading. This cyclical stress occurs in different components due to rotation, flexure or even vibration. This mode of failure is known as fatigue. Fatigue Testing: A specified mean load (which may be zero) and an alternating load are applied to a specimen and the number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. Loads may be applied axially, in torsion, or in flexure.
What are the three stages of fatigue in materials?
- Initially a tiny crack commences at a surface after continual cyclical loading. 2. The crack then gradually propagates as the load cycling continues. 3. Finally a sudden and catastrophic failure occurs when the remaining cross-section of the material is insufficient to carry the applied load.
What type of polymer material might you select for the following applications? Surgeons glove A pulley
SOLUTION a) Surgeons glove The glove must be capable of stretching a great deal in order to slip onto the surgeons hand, yet it must conform tightly to the hand to permit the maximum sensation of touch during surgery. A material that undergoes a large amount of elastic strain – particularly with relatively little applied stress – might be appropriate. This requirement describes an elastomer. b) A pulley: The pulley must be subjected to some stress and wear as a belt passes over it. A relatively strong, rigid, hard material is required to prevent wear. A thermosetting polymer might be most appropriate.
An amorphous polymer is pulled in a tensile test. After a sufficient stress is applied, necking is observed to begin on the gauge length. However, the neck disappears as the stress continues to increase. Explain this behaviour.
Normally, when necking begins, the smaller cross-sectional area increases the stress at the neck and necking is accelerated. However, during this tensile test, the chains in the amorphous structure are straightened out and the polymer becomes more crystalline. When necking begins, the chains at the neck align and the polymer is locally strengthened sufficiently to resist further deformation at that location. Consequently, the remainder of the polymer, rather than the necked region, continues to deform until the neck disappears.
Describe giving examples, what factors affect the inherent viscosity of a polymeric material.
• Molecular Architecture • Molecular chain length • Molecular weight distribution • Additive system employed
Describe TWO methods for the determination of flow behaviour in a polymer. What advantages and disadvantages do these methods have in a polymer manufacturing environment?
1) Capillary Rheometer The basic principle is that a thermoplastic sample (originally in the shape of granules, powder or flakes) is made fluid by heating and forced to flow out of a cylinder through a capillary die. The measured quantity is normally the generated pressure under steady state conditions. A flow curve is the typical output, obtained by interpolation of several experimental data. 2) Melt Flow Indexer It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures
Identify the parts of a thermoplastic extruder
- Motor and gearbox 2. Hopper 3. Water cooling system 4. Heater bands 5. Thermocouples 6. Screw 7. Screen pack 8. Breaker plate 9. Adapter zone 10. Die
(a) Describe the polymer manufacturing process of extrusion. (b) Identify the typical properties which can be determined using thermo-analytical techniques and why these are important?
(a). The extruder is essentially a screw conveyor: Carries cold plastic granules (or powder) forward Compacts them under pressure at high temperature Feeds the material forward into the form shaping die as a uniform and homogeneous melt (b) Thermo-Analysis (studying polymer properties as they change with temperature) Differential scanning calorimetry (DSC) Chemical changes Melt Thermogravimetric analysis (TGA) Physical changes: Weight loss and gain Moisture storage Degradation Thermomechanical analysis(TMA) Expansion/contraction in the mould Mould design etc By carrying out these types of tests, you are attempting to simulate manufacturing processes. Determine times polymer can be in the barrel or mould before changes take place. Differences in off-line / industrial techniques. Helps reduce development time for new materials/products
What factors of the in-coming raw polymeric material need to be assessed for quality control purposes?
To gain the most comprehensive understanding of the raw material, the following should be assessed: Flow characterisation under simulated processing conditions Thermal response Granule size variation Residue content, e.g. By ashing or TGA Note that the cost implications of this can be quite significant.
Describe with the aid of diagrams the three zones in an extruder and the importance of each for the manufacture of polymeric products.
- Feed Zone • Feeds solids forward • Packs material • Melt film begins to form (gel point) 2. Compression Zone • Flight depth decreases • Pressure increases • Compaction of polymer • Entrapped air squeezed out via hopper • High pressure to control flow 3. Metering Zone • Melt homogenisation • Uniform flow at constant temperature and pressure
Briefly discuss the important features of the die design in the extrusion process.
The important features in die design are : • The adapter and die system must give a smooth flow of melt with no dead spots. • The approach channel to the final parallel should taper gradually to maintain compression and assist flow. • The die parallel must be long enough to exert back pressure to control uniform flow. • The die faces must be aligned precisely.
What is the purpose of the breaker plate and screen pack system on an extruder?
The breaker plate and screen pack system are supported by the adapter zone clamped against the barrel. The screen pack: 1. Acts as a filter for coarse particles and contaminants. 2. Creates a back-pressure without which control of the flow of the melt would not be possible; this flow control is essential for uniform production. The breaker plate serves the double function of: 1. Supporting the filter pack, and preventing the fine mesh wires breaking under the pressure developed in the melt by the screw. 2. Breaking up the rotational flow of the melt, converting it into translational flow into the adapter zone
How does melting take place in an extrusion process?
- Melting begins at the end of the feed zone. 2. Thin film of molten polymer forms at the barrel wall (heat source). 3. Advancing screw flight scrapes molten film off wall. 4. Melt pool accumulates in front of the advancing flight (high pressure side). 5. Solid bed accumulates behind advancing flight (low pressure side). 6. As material is transported downstream melt pool increases in size at the expense of the solid bed. 7. Melting should be completed before entering the metering zone. Melting takes place due to: 1. Transfer of heat from the barrel walls. 2. Dissipation of mechanical energy into heat through the deformation of the solid plastic.
Discuss the effect of heating and cooling (fast and slow) in the polymer melt and the significance of these factors on the final product.
Polymers are inherently poor conductors of heat, having very low thermal conductivities, and as such are vulnerable to localised overheating which can lead to degradation (chain scission). Most commercial polymers contain anti-oxidants to minimise this effect. The removal of heat, i.e. the rate of cooling, will have a direct bearing on the structure and crystallinity of the product. In turn, these can influence the mechanical properties, the elastic recovery and shrinkage potential both during processing and in-service. Too much heat will degrade the material. Too slow a removal of heat will result in a highly crystalline structure, which will be difficult to process subsequently.
(a) Identify the major components of an injection-moulding machine. (b) Discuss the effect and type of defects in injected moulded parts.
(a). Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. Injection moulding machine consists of two major components: Injection unit The mould assembly and clamping unit (b) Shrinkage and mould defects Short shots – polymer solidification prior to the mould being filled. Flashing – excess polymer squeezed out at the parting line in the tool. Sink marks and voids – for thick mould sections because of the specific volume change during cooling if the solidified skin is thin then the internal stresses cause the surface to deflect causing a depression on the mould surface to accommodate the volume change. On the other hand if the skin is stiff and resists deflection then the volume change due to cooling must be accommodated internally and hence voids are formed. Weld lines – when polymer is injected into a mould at more than one point a number of flow fronts move throughout the cavity until ultimately these meet up. The flow fronts on confrontation move transversely (parallel) to each other with no mass flow across the boundary forming a weakness between the two fronts. Under load this can be readily fractured. Careful component and tool design is required to ensure that this is not situated in a critical part of the component.
Discuss the moulding assembly and clamping arrangement for an injection moulder.
Mould assembly is made up of at least two platens, which support the precision- engineered tool (mould). One half of this is movable so that the mould can be opened and closed to eject the solidified mouldings (components). The clamping unit can be toggle, hydraulic or hydromechanical mechanisms providing high pressures to resist the high injection pressures associated with this process, keeping the mould closed so that flash free components can be produced which require minimum trimming after ejection.
Discuss how defects occur during the injection moulding process
Short shots – polymer solidification prior to the mould being filled. Flashing – excess polymer squeezed out at the parting line in the tool. Sink marks and voids – for thick mould sections because of the specific volume change during cooling if the solidified skin is thin then the internal stresses cause the surface to deflect causing a depression on the mould surface to accommodate the volume change. On the other hand if the skin is stiff and resists deflection then the volume change due to cooling must be accommodated internally and hence voids are formed. Weld lines – when polymer is injected into a mould at more than one point a number of flow fronts move throughout the cavity until ultimately these meet up. The flow fronts on confrontation move transversely (parallel) to each other with no mass flow across the boundary forming a weakness between the two fronts. Under load this can be readily fractured. Careful component and tool design is required to ensure that this is not situated in a critical part of the component.
What are the three types of composites? – Give examples of each.
- Particulate: Concrete, cemented carbide (tungsten carbide particles in cobalt) 2. Fibrous: Wood, bone, glass fibre reinforced polymer, carbon fibre reinforced polymer 3. Laminate: Plywood
What is the definition of a composite material?
A composite material can be defined as: “A material system comprised of two or more physically distinct phases whose combination produces aggregate properties which are different and indeed superior to its constituents.”
What is the role of the textile in a composite material?
Textile reinforcements using engineering fibres for composites have good tensile strength and are lightweight but have poor performance in terms of compression or stiffness. This necessitates the use of a matrix to encapsulate the fibres, thus: Protecting them from damage (mechanical and/or environmental) Enhancing the performance of the composite, in particular overcoming some of the weaknesses of textiles.
What is the definition of a structural composite?
Structural composites can be defined as products which: Use fibre reinforcements; e.g. carbon, aramid or glass, 50-70% by weight Very high strength and stiffness Made with polymeric metal and other matrices The matrix binds the reinforcing fibres together, forming a cohesive structure. Applied stresses transferred from one filament through the matrix to the adjacent filament. Polymeric matrices give low densities with very high specific properties i.e. high strength/weight and high stiffness/weight ratios.
